//合并两个有序链表 -- 递归练习

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
        if(list1 == nullptr) return list2;
        if(list2 == nullptr) return list1;
        if(list1->val <= list2->val) 
        {
            list1->next = mergeTwoLists(list1->next, list2);
            return list1;
        }
        else
        {
            list2->next = mergeTwoLists(list2->next, list1);
            return list2;
        }
    }
};